Reduced RC delay between adjacent substrate wiring lines

ABSTRACT

A void is defined between adjacent wiring lines to minimize RC coupling. The void has a low dielectric value approaching 1.0. For one approach, hollow silicon spheres define the void. The spheres are fabricated to a known inner diameter, wall thickness and outer diameter. The spheres are rigid enough to withstand the mechanical processes occurring during semiconductor fabrication. The spheres withstand elevated temperatures up to a prescribed temperature range. At or above a desired temperature, the sphere walls disintegrate leaving the void in place. For an alternative approach, adjacent wiring lines are &#34;T-topped&#34; (i.e., viewed cross-sectionally). Dielectric fill is deposited in the spacing between lines. As the dielectric material accumulates on the line and substrate walls, the T-tops grow toward each other. Eventually, the T-tops meet sealing off an internal void.

This is a continuation of application Ser. No. 08,550,916 filed Oct. 31,1995, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to wiring line formation and inter-line fillprocesses for a semiconductor substrate, and more particularly, tointer-line fill processes for reducing RC delay between adjacent wiringlines.

Integrated circuit substrates include many different p-type and n-typedoped regions. These regions are connected in specific configurations todefine desired devices and circuits. Conductive paths are defined on thesubstrate to connect the various doped regions to form the many devicesand circuits. These paths typically are referred to as wires,interconnects, metal stacks, or conductors. The term "wiring line" isused herein to refer to all such conductive paths.

As device and circuit densities increase due to advances in technology,it is desirable to decrease wiring line pitches and spacings. A wiringline has a length, a thickness and a width. The non-line area betweenadjacent lines is referred to as the line spacing. The width and spacingis conventionally referred to as the line pitch. The spacing can bebetween lines on the same plane of the substrate or between lines onadjacent planes. Conventional line spacing of approximately 1.0 micronis known. There is a desire, however, to decrease line spacing as ICdevice densities increase.

One of the challenges of semiconductor processes is to maintainelectrically-independent wiring lines. Electrical coupling betweenadjacent lines is undesirable. Reliable, uncoupled signals carried alongadjacent lines are needed for normal circuit operation. One of thecoupling characteristics between adjacent lines is the RC delay ("RCcoupling"). Zero delay is ideal. Minimal RC delays are desired. As thespacing between two adjacent lines decreases, the RC coupling tends toincrease. One of the physical characteristics defining RC delay (otherthan spacing) is the dielectric value of the fill material in thespacing between adjacent lines. Currently, dielectric values ofapproximately 3.0 are common for 1.0 micron line spacing. A dielectricof approximately 3.0 is achieved using tetra ethyl oxy silicate ("TEOS")as the fill material between adjacent lines. Use of a high densityplasma oxide fill at the 1 micron spacing has been found to achievedielectric values between 2.4 and 2.7.

As the line spacings decrease (e.g., below 0.5 microns), new fillprocesses and materials are needed to avoid RC coupling and achieveminimal RC delays.

SUMMARY OF THE INVENTION

According to the invention, a void is defined between adjacent wiringlines to minimize RC coupling. The void has a low dielectric valueapproaching 1.0. The void is space absent solid and liquid material. Invarious embodiments the space is a vacuum or is filled with gaseoussubstance having desired dielectric properties.

According to one method of the invention, a hollow silicon dioxidesphere defines the void. The sphere is fabricated to a known innerdiameter, wall thickness and outer diameter. Preferably, the wiring lineheight is a multiple of the line spacing, or the spacing is a multipleof the wiring line height. Spheres of a unit dimension then fill thespacing to achieve one or more rows (or columns) of spheres.

According to one aspect of the method, the spheres are rigid enough towithstand the mechanical processes occurring during semiconductorfabrication.

The spheres are held in place during the semiconductor fabricationprocesses by a binder. According to another aspect of the method, thespheres and binder withstand elevated temperatures up to a prescribedtemperature range. At or above a desired temperature, the binder isbaked away leaving the sphere intact and in place.

According to another method of the invention, the adjacent wiring linesare "T-topped" (i.e., viewed cross-sectionally). In a specificembodiment the cross section appears as a "T" or as an "I." Dielectricfill is deposited in the spacing between lines by a chemical vapordeposition ("CVD") or other deposition process. As the dielectricmaterial accumulates on the wiring line and substrate walls, the T-topsgrow toward each other. Eventually, the T-tops meet sealing off aninternal void. Using controlled processes, the void is reliably definedto a known size and shape.

According to preferred embodiments a spacing between adjacent wiringlines of a semiconductor substrate includes a first material whichdefines a void. The void has no solid material or liquid material, butmay include a gas. Also, the void is characterized by a dielectricconstant which is lesser than the dielectric constant of the firstmaterial. In one embodiment a plurality of discrete hollow objects fillthe spacing. Each one of the plurality of objects is a hollow, rigid,silica sphere which defines a void. Each sphere is of substantially thesame dimensions. The spacing between adjacent lines is approximately afirst multiple of sphere outer diameter. The height of the adjacentwiring lines is approximately a second multiple of sphere outerdiameter. Preferably, either one, but not both, of the first multipleand the second multiple are greater than one.

According to a preferred embodiment of one method, a void iscontrollably-defined in spacing between adjacent wiring lines of asemiconductor substrate. At one step a plurality of discrete hollowsilica spheres are applied to the spacing. At another step excessspheres are removed from areas other than the spacing. At another stepmaterial is deposited over the wiring lines and spheres. For one methodthe spheres are applied as part of a film including a binder. The binderholds the objects in place within the spacing. For one method the excessspheres are removed by performing a chemical-mechanical polishing("CMP") process. Preferably, the deposition step occurs at a temperaturesufficient to break down the binder while leaving the spheres in placeand intact.

According to another preferred embodiment, a void iscontrollably-defining in spacing between adjacent wiring lines of asemiconductor substrate using an alternative method. At one step a T-topconfiguration is etched at each of the adjacent wiring lines. At anotherstep dielectric material is deposited onto the substrate and adjacentwiring lines. The deposited material accumulates about the T-tops toseal off a void in the spacing. The void forms with dimensionsdetermined by the spacing, wiring line height, and undercut of theT-tops. For various alternatives, the wiring line cross-sections afterT-topping resemble an "I" or a "T" configuration.

According to one advantage of the invention the controllably-definedvoid(s) reduce the dielectric value in the spacing between adjacentwiring lines. As a result, the RC delay is comparatively reduced.According to another advantage, the reduced dielectric is achieved forconventional (e.g., ≧1.0 microns) or reduced line spacings (e.g., <1.0microns; <0.5 microns). With sphere outer diameters achieved at 0.1microns, the method has the advantage of being beneficial for linespacing as low as 0.1 microns. As technologies enable smaller spheres,the method also becomes applicable for smaller line spacings. These andother aspects and advantages of the invention will be better understoodby reference to the following detailed description taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial cross-sectional view of adjacent wiring lines on asubstrate having a controllably-defined void in the line spacingaccording to an embodiment of this invention;

FIG. 2 is a partial cross-sectional view of adjacent wiring lines on asubstrate having a controllably-defined void in the line spacingaccording to another embodiment of this invention;

FIG. 3 is block diagram of a row of hollow spheres filling the spacingbetween adjacent wiring lines according to one embodiment of thisinvention;

FIG. 4 is a cross sectional view of a sphere of FIG. 3;

FIG. 5 is a cross-sectional view of a substrate receiving a film ofspheres according to a step of one method embodiment of this invention;

FIG. 6 is a cross-sectional view of a substrate afterchemical-mechanical polishing according to a step of one methodembodiment of this invention;

FIG. 7 is a cross-sectional view of a substrate in which wiring lineheight is a multiple of line spacing;

FIG. 8 is a cross-sectional view of a substrate after a layer isdeposited over the wiring lines and spheres of FIG. 6 according to astep of one method embodiment of this invention;

FIG. 9 is a cross-sectional view of a substrate with adjacent "T" shapedand "I" shaped metal wiring line stacks according to an embodiment ofthis invention;

FIG. 10 is a cross-sectional view of a "T" shaped metal wiring stack andan "I" shaped metal wiring stack of FIG. 9;

FIG. 11 is a cross-sectional view of the substrate of FIG. 9 duringdielectric deposition according to a step of a method embodiment of thisinvention;

FIG. 12 is a cross-sectional view of the substrate of FIG. 9 afterdielectric deposition according to a step of a method embodiment of thisinvention; and

FIG. 13 is a cross-sectional view of the substrate of FIG. 9 afterplanarizing according to a step of a method embodiment of thisinvention.

DESCRIPTION OF SPECIFIC EMBODIMENTS

Overview

FIG. 1 shows a partial view of an integrated circuit (IC) 10 having avoid 12 formed between adjacent wiring lines 14, 16. The wiring lines14, 16 are conductively coupled to respective portions of asemiconductor substrate 18. The substrate 18 includes various n-type andp-type doped regions interconnected by wiring lines, such as lines 14,16. The interconnected substrate regions define desired semiconductordevices. The wiring lines are formed by one or more layers, including abarrier layer and a metal conductive layer. The barrier layer serves toprevent metal penetration into the substrate 18 during the formationprocesses. The conductive layer couples a local area of the substrate(e.g., doped or not doped area) to another area (not shown) or anotherlayer 20. The spacing between adjacent wiring lines is occupied by fillmaterial 22, 24 or is a void 12.

FIG. 2 shows a partial view of an integrated circuit (IC) 30 having avoid 32 formed between adjacent wiring lines 34, 36 according to analternative embodiment of this invention. The wiring lines 34, 36 areconductively coupled to respective portions of a semiconductor substrate38. The substrate 38, like substrate 18 includes various n-type andp-type doped regions interconnected by wiring lines. The interconnectedsubstrate regions define desired semiconductor devices. The wiring linescouple a local area of the substrate (e.g., doped or not doped area) toanother area (not shown) or layer 40. The spacing between adjacentwiring lines are occupied by fill material 42 and the void 32. In theembodiment shown vias are formed through the fill material 42 and filledwith conductive material 44 to respectively couple the wiring lines 34,36 to an adjacent layer 40.

For the various IC embodiments a void 12/32 in the spacing betweenadjacent wiring lines serves to reduce RC coupling of the lines. RCcoupling is reduced by reducing the dielectric constant in the spacing.Specifically, because the dielectric constant of the void (e.g.,approximately 1.0) is less than the dielectric constant of conventionalfill materials (e.g., approximately 3.0), the dielectric constant in thespacing between lines is reduced.

Following are descriptions of alternative methods forcontrollably-defining the voids 12, 32.

Void Defined by Hollow Silicon Spheres

Referring to FIG. 3, a semiconductor substrate 10 having integraldevices (not shown) and wiring lines 14, 16 formed by known processesreceives hollow spheres 50. As seen in FIG. 4, the spheres 50 have aninner diameter 52, outer diameter 54 and wall thickness 56 of knowndimensions. In a preferred embodiment the sphere walls 58 are formed ofsilica. For a given embodiment, each sphere 50 has the same dimensions.Preferably the line height and the line spacing is a multiple of thesphere outer diameter. Alternatively, the sphere outer diameter isslightly less than a value which makes the spacing or height a multipleof the outer diameter. Although the outer diameter of each sphere 50 issubstantially the same for a given embodiments, the outer diametervaries for different embodiments. The outer diameter varies amongdifferent embodiments from a value greater 1.0 microns to a value lessthan 0.5 microns. Spheres as small as 0.1 microns in outer diameter areachievable.

The spheres 50 are of sufficient rigidity to withstand the mechanicalstresses occurring in fabricating an integrated circuit. In oneembodiment the ratio of outer diameter to wall thickness isapproximately 10:1, although greater or lesser ratios are used in otherembodiments.

At one step the spheres 50, together with a binder material and/ordispersion chemical, are applied to the substrate 10 using a spinningprocess or a monolayer formation process. An exemplary binder materialis methyl isobutyl ketone ("MIBK"). The function of the binder is tohold the spheres in place relative to the wiring lines 14, 16 andsubstrate 10. Exemplary dispersion chemicals include polyethylene oxideor a silanol compound. The function of the dispersion chemicals are todisperse the spheres into the line spacings and over the wiring linesand substrate. A film 60 formed by the spheres 50, binder materialand/or dispersion chemical accumulates on the substrate 10 and wiringlines 14,16 as shown in FIG. 5.

At another step the substrate is planarized. A chemical-mechanicalpolishing ("CMP") or other planarizing process removes the film 60 fromthe tops of the wiring lines 14, 16 as shown in FIG. 6. In oneembodiment the wiring lines 14, 16 have a height relative to thesubstrate 10 surface which is a multiple of the sphere 50 outer diameter54. For minor variations of height to outer diameter, the wiring lines14, 16 are planed back to be a multiple of sphere 50 outer diameter 54.For areas 62 not to be filled with the film 60, an etching process isused to remove the film 60 (see FIG. 6).

FIG. 6 shows two preferred relations between wiring line 14, 16 heightand wiring line spacing. In one region 64 the wiring line height equalsthe wiring line spacing. In another region 66 the wiring line spacing isa multiple (e.g., 2) of the wiring line height. Preferably the ratio ofthe longer of the height and spacing to the shorter of height andspacing is an integer, (i.e., either the spacing is a multiple of theheight or the height is a multiple of the spacing). FIG. 7 shows theheight being a multiple of the spacing. For the best mode of theinvention, spheres are applied which have an outer diametersubstantially equal to (or slightly smaller than) either one or both ofthe line height or the line spacing. In other embodiments either one orboth of the height and spacing are multiples of the sphere outerdiameter. Preferably both the line height and line spacing are not amultiple greater than 1 relative to the sphere outer diameter.

In alternative embodiments, either a dielectric or a plasma oxide layeris applied over the wiring lines 14, 16 and spheres 50. For dielectriclayer 68, low temperature dielectric reflow is deposited on the wiringlines 14, 16 and spheres 50. Reflow improves filling of highaspect-ratio contacts and via openings. Preferably, the depositionprocess occurs at a temperature high enough to bake off the bindermaterial, but low enough not to alter the structural integrity of thespheres 50. More specifically, one does not want to collapse or puncturethe spheres 50 during the dielectric reflow deposition step. In oneembodiment binder material capable of withstanding temperatures up to adesired temperature (e.g., 200 degrees C.) are used. Above the desiredtemperature the binder breaks down and flows out as a vapor, but leavingthe spheres in place and intact.

Alternatively for a plasma oxide layer 70, plasma oxide is depositedover the wiring lines 14, 16 and the spheres 50. Preferably, the processoccurs at a temperature sufficient to bake off the binder material,while leaving the spheres in place and intact. Further semiconductorprocesses then occur to fabricate another device level or area of thesubstrate 10.

Void Defined by Controlled Deposition

Referring to FIG. 9, a semiconductor substrate 38 has integral devices(not shown) formed by known processes. Metal stacks 72, 73 are formed todefine wiring lines 34, 36, 74, 76. According to alternative embodimentsthe stack cross-section appears as a "T" (e.g., stack 72) or an "I"shape (e.g., stack 73). Of significance is the "T-top" in eachembodiment. By depositing dielectric the T-tops of adjacent wiring linesgrow toward each other sealing off a void between adjacent wiring lines(see FIG. 2).

Referring to FIG. 10 each metal stack includes a barrier layer 78, aconductive layer 80 and a top layer 82. A common material for anexemplary barrier layer 78 is titanium, although other elements andalloys are used, (e.g., titanium nitride, titanium tungsten, tantalumnitride). A common material for an exemplary middle layer 80 isaluminum, although other elements and alloys also can be used, (e.g.,copper, gold). A common material for an upper layer 82 is titaniumnitride, although other materials and alloys are used, (e.g., titaniumtungsten, titanium, titanium aluminide, tantalum nitride).

In one embodiment the three layers are deposited then etched using areactive ion etching (RIE) process to achieve a straight metal stack.For an "I" stack 73 the conductive middle layer 80 is etched using a wetdip process to achieve the "I" configuration. For a "T" stack 72 boththe conductive middle layer 80 and the barrier layer 78 are etched usinga wet dip process to achieve the T-top configuration. Alternatively, thebarrier layer 78 and middle layer 80 are formed to desired shape by anRIE process. An isotropic overetch then is performed to achieve the"T-top" for either the "T" stack 72 or "I" stack 73.

For each stack 72, 73 configuration the length of undercut 84 isprescribed based upon a desired line resistance, the desired linespacing between adjacent stacks 72 and/or 73 and the size of voiddesired between adjacent wiring lines 34/36/74/76.

With the stacks formed at desired locations with desired dimensions(e.g., line height, pitch, undercut) and desired line spacingsdielectric material 86 is deposited using a CVD or other depositionprocess. Exemplary dielectric materials include TEOS, polyamide, Si₃ N₄,SOG, phosphosilicate glass, and boro-phosphosilicate glass. Thedielectric material 86 accumulates on the wiring lines 34, 36, 74, 76and substrate 38 as shown in FIG. 11. As the deposition processcontinues the dielectric material accumulating at adjacent "T-tops"seals off an area between the adjacent lines. Such sealed off area isthe desired void 32 (see FIGS. 2 and 12). The deposition processcontinues for a prescribed time or a prescribed thickness of dielectricmaterial accumulates above the wiring lines 34, 36, 74, 76. Thereafterthe substrate is subjected to a chemical-mechanical polishing process orother planarizing process to achieve a dielectric layer of desiredthickness, (see FIG. 13).

For embodiments in which vias 44 (see FIG. 2) are desired, a plasmaenhanced chemical vapor deposition of a nitride compound is deposited(e.g., approximately 100 angstroms) prior to dielectric deposition toserve as an etch-stop layer.

The formation of the voids 32 is controlled for a given line spacing by(i) appropriately defining the wiring line height 83 and under cut 84and (ii) controlling the deposition process. As a result, the voids 32occur with known size and shape. Voids 32 formed between adjacent "T"stacks are generally uniform in size and shape. Similarly, voids 32formed between adjacent "I" stacks are generally uniform in size andshape. The length of each void 32 is determined by the wiring linelength of adjacent wiring lines 36, 36, 74, 76.

The voids 32 have a dielectric constant of approximately 1.0. Thesurrounding dielectric material has a higher dielectric value (e.g.,TEOS has a dielectric constant of 3.0, high density plasma oxides have adielectric constant of 2.4-2.7). The net effect of the void is to lowerthe dielectric constant across the line spacing and thereby reduce RCcoupling between adjacent lines.

Further semiconductor processes also occur after void formation tofabricate additional devices, levels or other area of the substrate 38.

Meritorious and Advantageous Effects

According to one advantage of the invention the void in the spacingbetween adjacent wiring lines reduces RC coupling of the lines. RCcoupling is reduced by reducing the dielectric constant in the spacing.Specifically, because the dielectric constant of the void (e.g.,approximately 1.0) is less than the dielectric constant of conventionalfill materials (e.g., approximately 3.0), the dielectric constant in thespacing between lines is reduced. According to another advantage, thereduced dielectric is achieved for conventional or reduced linespacings.

Although a preferred embodiment of the invention has been illustratedand described, various alternatives, modifications and equivalents maybe used. Therefore, the foregoing description should not be taken aslimiting the scope of the inventions which are defined by the appendedclaims.

What is claimed is:
 1. A method for controllably-defining a void inspacing between adjacent wiring lines of a semiconductor substrate,comprising the steps of:applying a plurality of discrete hollow objectsto the spacing, each one of the plurality of objects comprising a firstmaterial and defining a void; removing said objects from areas otherthan the spacing; and depositing second material over the wiring linesand plurality of objects.
 2. The method of claim 1, in which the step ofapplying comprises applying a film, the film comprising the plurality ofdiscrete hollow objects and a binder for holding the objects in placewithin the spacing.
 3. The method of claim 2, in which the step ofremoving comprises performing a chemical-mechanical polishing process todefine a planar surface comprising the adjacent wiring lines and film.4. The method of claim 3, in which the step of depositing occurs at atemperature sufficient to break down the binder while leaving theplurality of objects in place and intact.
 5. The method of claim 1 inwhich each one of the plurality of objects is a sphere of substantiallythe same dimension.
 6. The method of claim 5 in which spacing isapproximately a first multiple of sphere outer diameter and wiring lineheight is approximately a second multiple of sphere outer diameter.